Natural gas, refinery gas, carbon dioxide, hydrogen, synthesis gas, gas from an oil production facility and other industrial gases are often used in circumstances that require the water in these gases to be removed. Water may be removed, for example, to prevent the formation of hydrates in downstream processes and pipelines, meet dew point specifications for the sale of the gas, and to prevent corrosion associated with wet gas.
There are two general categories of gas dehydration systems: solid desiccant and liquid desiccant. Liquid desiccant systems are relatively simple to operate and easy to maintain. Unfortunately, the liquid desiccant systems are typically unable to produce gases with extremely low levels of moisture. Solid desiccant systems are often used to provide gas with very low levels of moisture. However, these plants can be more complex and expensive to operate than liquid desiccant systems. Thus, there is a continuing need for a relatively simple liquid desiccant gas dehydration system that produces gas with the low moisture content normally associated with solid desiccant systems.
Hygroscopic liquids such as triethylene glycol, diethylene glycol and tetraethylene glycol are commonly used liquid desiccants. In the typical liquid desiccant system, a substantially dry glycol such as one of those listed above is introduced to the top of a contactor. The liquid desiccant flows downward through the contactor while at the same time wet gas is introduced at the bottom of the contactor. When the liquid desiccant and gas contact each other, the liquid desiccant absorbs water from the gas. Water-rich liquid desiccant is removed from the bottom of the contactor, while dry gas leaves the top of the contactor. Water absorbed by the liquid desiccant is removed by the application of heat and the liquid desiccant is thus regenerated and reused. The dryness of the gas, expressed as its “dew point,” depends on several factors, including the water content of the dry liquid desiccant, the number of theoretical stages in the contactor, and the liquid desiccant and gas flow rates. The dew point of the dry gas leaving the contactor decreases as the water content of the dry liquid desiccant entering the contactor decreases. To produce a dry gas with a very low dew point, it is essential that the dry liquid desiccant entering the contactor have extremely low moisture content.
The regeneration of wet liquid desiccant is typically accomplished by heating it in order to vaporize the water it has absorbed from the wet gas. The concentration of water in a regenerated liquid desiccant depends in part on the regeneration temperature and pressure. Theoretically, it is possible to produce liquid desiccant with very low levels of water by subjecting the liquid desiccant to high temperatures. However, as the regeneration temperature approaches the boiling point of pure liquid desiccant, the liquid desiccant thermally decomposes. To avoid this problem, thermal regeneration of liquid desiccants is usually limited to temperatures below the thermal decomposition point of the liquid desiccant. This results in a relatively high concentration of water in the regenerated liquid desiccant. The higher concentration of water in the liquid desiccant produces dry gas with a higher than desirable dew point. To date, attempts to deal with the problem of producing low dew point gas with a liquid desiccant system have met with limited success.
A number of processes have been developed which seek to achieve the very low dew point levels achievable with solid desiccant systems which use silica gel, alumina or molecular sieves using liquid desiccants such as tri-ethylene glycol or di-ethylene glycol. These are generally referred to as “Enhanced Liquid Dehydration Systems”. The promise of such processes is that the lower energy and capital costs normally associated with liquid desiccant systems could be used advantageously in lieu of solid desiccant systems, for example, in cryogenic natural gas processing plants. Despite approximately thirty years since liquid desiccants have made the claim of being able to replace solid desiccant systems in these applications, such a revolution has never taken place. The enhanced liquid desiccant designs, while able theoretically to achieve water dew point levels experienced with the competing solid desiccant systems, are able to do so only with additional complexity and system sizing which detract from the advantages of these designs. Conventional enhanced liquid desiccant systems employ the principle of introducing an inert “stripping gas” into the regeneration system in order to lower the partial pressure of the water in the hot liquid desiccant. Generally, there have been two methods of providing stripping gas, each with its own advantages.
The original and most general method of reducing the water content of the desiccant was to take a small portion of the dried product gas and introduce it into the desiccant regenerator in counter-current flow into the bottom of a stripping tower which was used to dry the hot partially dried liquid leaving the heater/reboiler. Such a process is described in U.S. Pat. No. 3,105,748. This patent first described the supplementary stripping tower, referred to as a “super dryer”. This tower has remained a constant feature of almost all enhanced regeneration systems that followed. In U.S. Pat. No. 6,299,671, a very similar design is proposed which sources its stripping gas by recycling it from the regenerator overhead system. These systems suffer from the fact that the stripping gas must be recovered and returned to the operation since the gas will contain pollutants and have too much value to simply vent to the atmosphere.
One significant improvement in the design of enhanced liquid desiccant systems consisted of using a stripping gas which used vaporized liquid hydrocarbons produced as a side stream of the regeneration process. This design and its many improvements were commercialized under the name of “Drizo” and have a substantial operating record in the industry. These inventions are well described in U.S. Pat. Nos. 3,349,544 and 4,005,997. In these systems the “stripping gas” consists of the liquid hydrocarbons recovered by cooling, condensing and separating the water/hydrocarbon mixture exiting the top of the regenerator. Since these hydrocarbons are liquid at ambient conditions they do not need to be compressed as a gas to be returned to the plant. The liquids are vaporized, super heated and fed in a counter-current flow to the drying tower in a manner identical to the simpler stripping gas systems described in U.S. Pat. Nos. 3,105,748 and 6,299,671. The disadvantage of these vaporized liquid hydrocarbon systems stems from the fact that the liquids, which are used to strip the desiccant, are also water saturated since they are produced as the condensation product of the water effluent of the regeneration system. Accordingly, the presence of water in the stripping gas itself limits the degree of dehydration that can be achieved.
Finally U.S. Pat. No. 5,643,421 remediates certain limitations of, for example, U.S. Pat. No. 4,005,997, by adding a dry desiccant system to the regenerator. This system serves to dry the liquid hydrocarbon stream used as stripping gas before it is heated and vaporized. While such a system is functional it is so at the expense of a significant amount of additional equipment and operating expense.